Typically when making a lateral exit from a borehole, a whipstock is run in with an anchor and a mill assembly is attached above the whipstock. The whipstock is oriented at the proper depth in a variety of ways and once the orientation for the whipstock ramp is obtained, the anchor is typically set hydraulically. Typically, pressure is delivered through the running string and into the widow mill that is releasably secured to the top of the whipstock. The anchor has to be set before the mill assembly can be sheared loose from the whipstock ramp by applied axial force or rotation. In order to deliver the needed pressure at the anchor to set the anchor, the circulation ports in the window mill are typically isolated with a rupture disc. The pressure in the running string is built up to a first level to set the anchor. This is made possible by the rupture disc blocking the circulation ports in the window mill. After the anchor is set the pressure is further built up to break the rupture disc so that flow from the running string can exit the circulation ports in the window mill as the string is rotated to break the shearable support that connected the milling bottom hole assembly to the top of the whipstock ramp. The mills are then advanced and the whipstock ramp guides the window mill to start a lateral opening in the surrounding tubular that will then be extended into a lateral from a main bore.
What is a shortcoming of this design is that it is expensive. Not only is there a high cost for the rupture disc but the connection between the window mill and the hydraulic anchor that has to span the length of the whipstock has been in the past a braided hose which has limited pressure rating and is also very expensive. The limited pressure rating affected the available setting pressure for the hydraulic anchor.
Some typical examples of the dual pressure systems that built pressure to a first level with the window mill circulation ports isolated with a rupture disc and then raised pressure to a second level to break the rupture disc to make the window mill circulation ports operational are: U.S. Pat. No. 9,004,159 FIG. 4; U.S. Pat. No. 8,739,900 FIG. 8 and U.S. Pat. No. 8,997,895 FIG. 4.
The present invention makes it possible to set the hydraulic anchor without needing a rupture disc for the window mill circulation ports. The hydraulic line passes through a piston and is connected to that piston. The piston is held in position with one or more shear pins or the like and when the pressure has been increased to set the anchor and then further increased to break the shear pin or the like the piston moves into a larger bore to expose ports to allow flow into the window mill circulation ports as the piston itself is captured in the chamber. High pressure tubing is pushed forward with the piston until the piston travel ends. Pulling the running string upward after breaking the shear bolt that attaches the window mill to the whipstock will assist moving the piston to the bottom of the piston housing. Tubing extending from the cutting structure of the window mill is simply ground off when the window mill is released from the whipstock ramp and milling the window begins. These and other aspects of the present invention will be more readily apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawing while recognizing that the full scope of the invention is to be determined from the appended claims.